The authors measured osteon dimensions and number in specimens of cortical bone obtained from the medial femoral neck (calcar) from nine female patients treated with hemiarthroplasty for femoral neck fractures, and from 12 (seven women, five men) age-matched cadavers without fractures. Specimens from the same location in 14 (seven women, seven men) younger patients treated with total hip arthroplasty (THA) for osteoarthrosis were also studied. There were fewer osteons per unit area and the osteons and their Haversian canals were bigger in the fracture group than in the nonfracture or osteoarthritic groups. Patients with femoral neck fractures also exhibited a decreased Singh index and an increased intracortical porosity compared with age-matched normal controls. By contrast, the osteoarthritic group showed no difference in osteon or Haversian dimensions, osteon number per unit area, Singh index, or in porosity compared with the nonfracture group. These morphometric differences found in the cortical bone of the medial femoral neck may play a role in the incidence of fractures.